This invention is in the field of artificial joints and intervertebral prosthetics which are able to retain at least some degree of natural motion while providing shock absorption and improved durability.
Degenerative changes in the joints of the human body and the intervertebral discs of the human spine can begin in early adulthood. Aging, wear and tear, and disease, such as osteoarthritis, can cause severe damage and loss to cartilage within the joints, often times requiring joint replacement or joint arthroplasty surgery. This includes all joints within human anatomy such as spinal endplate joints, spinal facet joints, hip, knee, shoulder, elbow, and cranial joints. Similarly, disc degeneration, annular tearing, nucleus herniations, and internal disc disruptions can cause disc material to extrude out of the annulus altering the elastic properties of the intervertebral disc, and are treated with disc replacement or disc arthroplasty surgery. Disc degeneration is of particular concern because the blood supply to the intervertebral discs diminishes at adulthood, therefore there is no regeneration of cells.
In joint replacement surgery, one or more surfaces of an arthritic or damaged joint is removed and replaced with an artificial joint, typically called a prosthesis. Joint replacement, or joint arthroplasty, is becoming more common with over approximately 750,000 Americans having a hip or knee replaced each year. While hip and knee replacement surgeries may be the most common, joint replacement can also be performed on other joints such as the shoulders, fingers, and ankles.
The current standard for treating degenerative disc disease and other spinal disorders is a decompression and fusion. Intervertebral disc, bone, and other material are removed to increase the spaces in which the spinal cord and nerves travel through the vertebral column. Mechanical fixation such as pedicle screws and rods are then implanted for initial structural support. Bone growth is then induced by the addition of materials such as bone graft and bone morphogenic proteins. In many cases, the intervertebral disc is completely removed and an allograft or a synthetic cage is implanted in the intervertebral disc space to restore proper disc height and assist in the fusion process. Spinal fusions, however, are not always favorable treatments because fusions disrupt the natural bio-mechanics of the spine by eliminating motion at the fusion site. Spinal fusions also increase the strain on the remaining motion segments of the spine.
Artificial joints are also used in prosthetic limbs, such as U.S. Pat. No. 6,280,404 (“Morinaka”) and U.S. Pat. No. 5,139,526 (“Skardoutos”), where it is desirable for the joints to provide durability, shock absorption, and a range of motion similar to the natural body. Similarly, artificial joints are used in robotics where it is desirable to provide durability as well as movement similar to a human body, such as in surgical settings (U.S. Pat. No. 7,689,320 (“Prisco”)) and assembly lines (U.S. Pat. No. 7,975,568 (“Zhang”)).
There is a need for artificial joints, which include intervertebral prosthetics, that provide motion similar to the natural human body and have less of an effect on the bio-mechanics of the natural joint or spine if implanted into the body. Such joints must allow for rotation and torsion, translation, flexion, and extension, and lateral bending motion similar to that of a natural joint or intervertebral disc. Such implants must also ensure proper artificial joint height, prosthetic length, or disc height, and provide shock absorption similar to the compressibility of a natural joint or intervertebral disc.
Many previous artificial joint devices provide motion without shock absorption. To provide shock absorption, portions of the prosthetic, particularly the load bearing regions, can be coated with a compressible elastomer (see U.S. Pat. No. 3,938,198 (“Kahn”)). However, most elastomeric devices do not isolate motion of the device from the shock absorption of the device. Using the elastomer for shock absorption in addition to motion of the device requires a softer elastomer, which is more susceptible to breakdown than harder elastomers. Harder elastomers generally are less susceptible to breakdown and have a lower coefficient of friction but provide less shock absorption.
Elastomeric devices are particularly susceptible to breakdown at the interface between the elastomer and the endplate. Previous devices typically do not use low friction materials at the interface between the elastomer and the end plate prosthetic and instead cement or bond the surfaces at this interface. However, bonding does not disperse the force related to the translational motion between the surfaces. In addition, solvents used in bonding material can degrade the elastomeric bonds and weaken the elastomeric properties.
U.S. Pat. No. 4,231,122 (“Koeneman”) describes an artificial knee where the motion of the device does not occur at the interface of the elastomer and endplate. This device is then able to rely upon compression resilient elements made from elastomers to accommodate and carry the user's body weight. However, Koeneman notes that a pivot or pivotable assembly that incorporates a tubular body of elastomer in such a manner is only capable of extensive pivotal movement about a single axis. To provide some degree of rotation, this device requires two separate, parallel pivoting members to be implanted into the bone side by side. Each pivot member transfers the weight placed on the joint to a single pin as opposed to dispersing the weight across a wide surface. In addition, the elastomers are used as part of the motion limiter and as a foot, where both of these uses are not integrated with the motion of the device itself. Furthermore, the elastomers are disposed in between parts rather than casted into the device using negative spaces within the device.
U.S. Patent App. 2011/0004313 (“de Villers”) describes a motion core in a prosthetic disc that is configured to slide relative to upper and lower surfaces; however, inserting a resilient material within the core as described in “de Villers” limits the amount of shock absorbing material and is less effective than if extended further from the center of articulation. This device also does not provide stress relief at the interface between the resilient material and the upper and lower parts of the core where fracture of the material or the bond between them is likely to occur.
U.S. Pat. No. 7,156,876 (“Moumene”) describes a motion disc having an articulating core and a peripheral shock absorbing component; however, the “tenacious attachment” of the shock absorbing component to the endplate components has an ineffective interface where it is likely for the shock absorbing material to fracture or for the bonding between the surfaces to fail than if the surfaces were allowed to slide. In the embodiment without tenacious attachment, the articulation and shock absorption components are separate components in their motion, which increases stress on the shock absorbing material compared to combining these components in motion.
U.S. Pat. No. 6,936,071 (“Marnay”) describes an intervertebral implant that also has two structural components that engage the natural vertebral endplates, and a core component with concavity for articulation. This device is an example of a group of devices that provide articulation without any shock absorbing component.
U.S. Patent App. 2010/0324689 A1 (“Obrigkeit”) describes an artificial spinal implant comprising a thermoplastic elastomer that has a hard phase and soft phase. This invention uses a two phase elastomer to increase the durability of the elastomer; however, the device relies solely on the elastomer for movement. This device has a disadvantage because the entire load and movement of the device is transferred to the elastomer; thus, the elastomer is more susceptible to breakdown. This device also does not address the stress between the interface of the elastomer and the non-motion parts of the device.
U.S. Pat. No. 4,911,718 (“Lee”) describes an intervertebral disc spacer that is comprised of an elastomer core that is reinforced by a fibrous laminate that is intended to simulate the natural nucleus pulposus and annulus fibrosus. Similar to “Obrigkeit,” the entire load and movement of the device is placed on the elastomer and the device does not address the stress between the interface of the elastomer and the non-motion parts of the device. Also, “Lee,” uses layers of elastomer with different hardness but has a soft core rather than a rigid core.
U.S. Pat. No. 4,759,766 (“Buettner-Janz”) describes an intervertebral disc endoprosthesis comprising two concave end plates with an intermediate convex spacing piece. All three components have a plane guide rim. Similarly to “Marnay,” this device does not contain any compressive material for shock absorption.
Foreign Patent No. EP 2 377 495 A1 (“Mingyan”) describes a spinal implant that has two components, each with a surface that engages a vertebra and an interior surface that interfaces a core. Similarly to “Obrigkeit” and “Lee,” the motion and load is placed into the viscoelastic material. Similar to “Moumene,” there is an ineffective interface between the shock absorbing material and the endplate prosthetic. In addition, the viscoelastic material has no reinforcement such as a flange or a fibrous laminate as in “Lee.”
U.S. Pat. No. 7,731,753 B2 (“Reo”) describes upper and lower end plates separated by a compressible core member. The plates are wound together with high tensile strength fibers or other engagement mechanisms. In this device, the compressible core member is made of an elastomeric material, woven fabric, spring member or a combination of these materials. Similarly to “Obrigkeit,” “Lee,” and “Mingyan,” this device relies on the core material to preserve motion between the vertebral bodies. This device relieves the stress between the interface of the core and the end plate prosthetics by using a cylindrical shaped core that can flatten out rather than allowing the interface between the core and the endplate to slide with low-friction as in the present device.
U.S. Pat. No. 6,585,770 B1 (“White”) describes a device for supporting bony structures such as two vertebral bodies. This device is commonly used during procedures where a diseased vertebral body is partially or entirely removed and a cage is used to support the space between the vertebral bodies above and below the excised body. In contrast, in certain embodiments described herein, the present invention uses mesh cages for anchoring a prosthetic to bone or cartilage rather than supporting two bony structures.
Despite these advances in the field, there is still a need for more durable artificial joints and prosthetic implants that maintain motion and shock absorption and have less of an effect on the natural bio-mechanics of the human spine and joints. Previous inventions that use an elastomer for shock absorption do not contain a rigid internal structure for the elastomer, do not properly address the interaction between the elastomer and non-motion parts, and do not disperse the translational force at the interface between the elastomer and the endplate prosthetic using low-friction sliding.